A. Field of the Invention
This invention relates to a method for manufacturing magnetic recording media. More specifically, this invention relates to a method for manufacturing perpendicular magnetic recording media. Still more specifically, this invention relates to a method for manufacturing discrete track media or patterned media which have satisfactory electromagnetic conversion characteristics at high recording densities and excellent productivity.
B. Description of the Related Art
Magnetic recording devices are one type of information recording device supporting the information-oriented society of recent years. As the volume of information has increased, ever-higher recording densities for the magnetic recording media used in magnetic recording devices have been demanded. In order to realize higher recording densities, the units in which magnetization reversals occur must be made smaller. To this end, it is important that the sizes of magnetic grains be reduced while simultaneously clearly separating the units of magnetization reversal (recording units), so as to weaken magnetic interaction between adjacent recording units.
As technology to realize higher recording densities, perpendicular magnetic recording methods are being studied in place of conventional longitudinal magnetic recording methods. At present, CoCr system alloy crystalline films, having a hexagonal close-packed (hcp) structure, with crystal orientation controlled such that the c axis is perpendicular to the film plane (that is, the c plane is parallel to the film plane), are primarily being studied as materials of the magnetic recording layers used in media for perpendicular magnetic recording methods. Further, in order to accommodate further increases in recording density, finer crystal grains in such CoCr system alloy crystalline films, narrowing of grain diameter distributions (reduction of the variation in crystal grain size), weakening of the magnetic interaction between grains, and other issues are being studied.
The use of what is generally called a granular magnetic layer, which is a magnetic recording layer having a structure in which magnetic crystal grains are surrounded by a nonmagnetic nonmetallic material such as an oxide or nitride, is being studied as one method to further raise recording densities. In a granular magnetic layer, the grain boundary phase, formed by the nonmagnetic nonmetal material, physically separates the magnetic crystal grains, so that magnetic interaction between magnetic crystal grains is reduced. This suppresses the formation of zigzag domain walls arising in recording unit transition regions, and low noise characteristics are obtained. For example, a perpendicular magnetic recording medium has been proposed in which a magnetic recording layer of a CoPtCrO alloy with a granular structure is layered upon an Ru underlayer (see IEEE Trans., Mag., Vol. 36, 2393 (2000)). In this perpendicular magnetic recording media, as the thickness of the Ru layer which is the underlayer is increased, the c-axis orientation of the magnetic recording layer with a granular structure is enhanced. In other words, as the thickness of the Ru layer is increased, perpendicular magnetic recording media having excellent magnetic characteristics and electromagnetic conversion characteristics are obtained. Also, the use of RF sputtering deposition employing a CoNiPt target to which SiO2 or other oxides are added has been reported to produce a magnetic recording layer with a granular structure in which individual magnetic crystal grains are surrounded and separated by nonmagnetic oxides, so that magnetic recording media with low noise can be obtained (see U.S. Pat. No. 5,679,473).
Further, the provision of a crystal orientation control layer directly below a granular-structure magnetic recording layer, using a material having an hcp crystal structure similar to that of the magnetic recording layer material, has been proposed (see Japanese Patent Laid-open No. 2003-123239 and Japanese Patent Laid-open No. 2003-242623). In this configuration, Co grains grow in the magnetic recording layer at positions corresponding to crystalline regions (crystal grains) in the crystal orientation control layer, and oxides in the magnetic recording layer precipitate and grow at positions corresponding to crystal grain boundaries, porous regions, or amorphous regions in the crystal orientation control layer. In other words, the magnetic crystal grains in the magnetic recording layer grow epitaxially on crystal grains in the crystal orientation control layer, and by this means the crystal orientation of the crystal orientation control layer is reflected in the crystal orientation of the magnetic recording layer. Simultaneously, amorphous-phase crystal grain boundaries are formed on the periphery of the magnetic crystal grains in the magnetic recording layer. In this way, the crystal state in a granular-structure magnetic recording layer can be controlled.
Perpendicular magnetic recording media having comparatively good magnetic characteristics and electromagnetic conversion characteristics are obtained by using a granular-structure magnetic recording layer. However, the granular-structure magnetic recording layers in perpendicular magnetic recording media of the prior art have been continuous films (so-called full-coverage films). In order to further raise the recording densities of perpendicular magnetic recording media, it is necessary to prevent write bleeding in adjacent recording tracks, reduce the formation of zigzag domain walls due to randomly-positioned grains, alleviate the effect of thermal fluctuations due to reduced crystal grain sizes, and decrease the magnetic interaction between magnetic crystal grains.
Related to the need for the above improvements, discrete track media and patterned media have been proposed. Discrete track media are perpendicular magnetic recording media in which the magnetic recording layer is formed from a plurality of magnetic member strips, which are completely separated magnetically. By using the plurality of magnetic member strips as recording tracks, boundaries between adjacent recording tracks are artificially formed, and magnetization reversal units are clearly demarcated. In discrete track media, the above-described write bleeding to adjacent recording tracks, as well as the formation of zigzag domain walls due to randomly placed grains can be prevented. On the other hand, patterned media is an ultimate form of perpendicular magnetic recording media in which the magnetic recording layer is formed by artificially arranging the shapes and sizes of a plurality of “islands” forming unit magnetic domains, with each of these “islands” being used as a single magnetization reversal unit (recording unit or bit).
Various methods have been proposed for the manufacture of discrete track media and patterned media. For example, a method has been proposed in which etching is used to physically separate the magnetic recording layer (see Japanese Patent Laid-open No. 4-310621). As a modification of this method, a method has been proposed in which etching is used to remove a portion of the magnetic recording layer and the crystal orientation control layer there below, and pack the removed portions with a nonmagnetic material, to form a magnetic recording layer comprising a plurality of magnetically independent portions (see Japanese Patent Laid-open No. 2006-12285). Also, a method has been proposed in which etching is used to form depressions in the substrate surface which are filled with a magnetic recording layer (see Japanese Patent Laid-open No. 56-119934). A further method has been proposed in which a portion of a soft magnetic layer formed on a substrate is removed, and magnetic material is layered thereupon, to form a magnetic recording layer comprising a plurality of magnetically independent portions (see Japanese Patent Laid-open No. 1-158617). Finally, a method has been proposed in which a portion of a soft magnetic layer and a portion of a crystal orientation control layer formed on a substrate are removed, and magnetic material is layered thereupon, to form a magnetic recording layer comprising a plurality of magnetically independent portions (see Japanese Patent Laid-open No. 2003-16622).
As stated above, in methods of manufacture of discrete track media and patterned media proposed in the past, portions of either the magnetic recording layer, or the magnetic recording layer and crystal orientation control layer, or the substrate, or the soft magnetic layer, or the soft magnetic layer and crystal orientation layer, are intentionally removed, to form a magnetic recording layer comprising a plurality of magnetically independent portions (see Japanese Patent Laid-open No. 4-310621, Japanese Patent Laid-open No. 2006-12285, Japanese Patent Laid-open No. 56-119934, Japanese Patent Laid-open No. 1-158617, and Japanese Patent Laid-open No. 2003-16622).
However, when removing portions of the magnetic recording layer, or of the magnetic recording layer and crystal orientation control layer, direct etching of the magnetic recording layer itself is entailed. Consequently the magnetic characteristics of the magnetic recording media are degraded due to damage to the magnetic recording layer caused by etching, corrosion of the magnetic recording layer due to remnant components of the etchant, and other causes. Furthermore, when removing portions of the substrate, it is difficult to form a magnetic recording layer having satisfactory crystal orientation and perpendicular anisotropy in the minute depressions (grooves). Hence satisfactory magnetic characteristics for the magnetic recording media cannot be expected.
Moreover, when removing portions of the soft magnetic layer, or of the soft magnetic layer and the crystal orientation layer, a planarization process of packing nonmagnetic material into the depression thus formed and of using chemical-mechanical polishing (CMP) or similar to smooth the surface is necessary. This is because, when large depressions and protrusions exist in the surface, the magnetic head flying stability declines. However, it is difficult to pack, uniformly and without gaps, a minute depression which has a high aspect ratio (ratio of the opening dimension to the depth dimension). In particular, when the aspect ratio (ratio of the opening dimension to the depth dimension) is high and the depression is minute, there is the possibility that depressions and protrusions in the surface after packing may be increased, depending on the depressions/protrusions prior to packing. For this reason, even when CMP is applied to the surface after packing, obtaining a completely smooth surface is difficult. Also, the amount of polishing is increased, and there is the possibility that film thickness can no longer be controlled.
Hence there remains a demand relating to manufacturing methods for discrete track media and patterned media comprising a plurality of magnetically independent portions which do not give rise to the above-described problems. In particular, there is a demand relating to manufacturing methods for discrete track media and patterned media, enabling a magnetic recording layer having excellent magnetic characteristics to be obtained without imparting damage to the crystal orientation control layer which is at the surface when forming the magnetic recording layer.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.